The process technology for the manufacture of polypropylene (PP) has evolved with improvements in catalyst technology, from complex slurry processes using an inert hydrocarbon diluent, to simpler bulk processes using liquid propylene diluent, to even more simplified gas phase processes.
The polymerization catalysts conventionally employed in these processes have generally been Ziegler-Natta type catalysts. Typical catalyst systems include a high activity, magnesium halide supported, transition metal containing component, an aluminum alkyl component, and preferably an external modifier or electron donor component.
The physical properties of homopolymers and copolymers of propylene formed by typical Ziegler-Natta polymerization typically are dependent on the stereoregularity of the polymer itself. Highly stereoregular polymers are crystalline, have desirably high flexural moduli and display high melting points. The addition of various electron donor materials to Ziegler-Natta catalysts is known to influence the degree of stereoregularity in polypropylene homopolymers and copolymers. Generally, a Ziegler-Natta catalyst, such as, for example, a magnesium chloride-supported, titanium-based catalyst, can be used in combination with any of a number of electron donor materials, each of which will lead to a specific level of stereoregularity and melt flow rate (MFR) control.
The molecular weight, and thereby the MFR, of the polyolefin produced with a particular catalyst system typically is regulated by the hydrogen level in the reactor. One of the properties of electron donors is that the stereoregulating capability and hydrogen response of a given electron donor are directly and inversely related. This relationship between stereoregularity and hydrogen response poses a problem. When highly stereoregulating donors are employed, it is necessary to use a high reactor hydrogen level to produce polyolefin, for example, polypropylene, having a molecular weight and MFR falling within the range usually desired for a particular use. A more hydrogen-responsive, lower stereoregulating donor will provide polypropylene with the equivalent MFR at a lower hydrogen level, but the polypropylene will be less stereoregular, having increased amorphous polypropylene content. Thus, in processes that have a hydrogen partial pressure limitation, the final achievable MFR will be determined by the choice of electron donor, which in turn determines the level of polypropylene stereoregularity in the final product.
Olefin polymerization processes, in which homopolymer composition is controlled through sequential addition of two different electron donor materials, are disclosed in the art. Such processes are carried out using two polymerization reactors connected in series. Homopolymer is produced in the first reactor with a Ziegler-Natta catalyst system, including a first electron donor, and passed to the second reactor, where a second electron donor is added. Although the catalyst in the second reactor will thus include a mixture of two donors, the more stereoregulating donor will control the composition of the product. The second donor thus will be selected to be more stereoregulating than the first electron donor material. The hydrogen levels in each reactor, and thereby the MFR for the product of each polymerization step, may be controlled differently as desired.
The sequential use of two donors in this manner provides a homopolymer product mixture having a broad molecular weight distribution and a broad compositional distribution of the homopolymer components. The MFR of the homopolymer will be substantially that which would be predicted for a mixture comprising the weighted average of the independently produced donor products. However, the product characteristics will be closer to those of a homopolymer formed in the presence of the more stereoregulating second donor alone. That is, the crystallinity and flexural modulus of the resulting homopolymer are higher than expected from the weighted average of the two independent donor products.
Separate reactors are used for conducting prior art polymerization processes employing a Ziegler-Natta catalyst system and a plurality of external donors. When conducted in a single reactor, whether in solution or in bulk, or in the gas phase as a continuously stirred tank reactor or fluid bed process, product composition will be controlled by the more stereoregulating external donor, also termed the dominant donor. For example, as disclosed in U.S. Pat. No. 6,111,039, incorporated in its entirety herein by reference, bulk liquid polymerizations conducted in a single stirred reactor using a mixture of two external donors produce polymer with a molecular weight distribution and melt flow rate very near that obtained using the dominant donor alone, even when as little as 10% of the dominant donor is present. The more stereoregulating donor is less hydrogen-responsive; hence, higher reactor hydrogen levels are needed to control molecular weight when such donors are present. Obtaining a product mixture containing the desired level of the product of the less stereoregulating donor having the desired molecular weight thus has heretofore required the use of separate reactors. U.S. Pat. No. 6,111,039 and PCT Published Application WO 99/20663 describe using different donors in different stages of an olefin polymerization process.
Although the use of a plurality of electron donors allows control of tacticity, molecular weight distribution and MFR in propylene polymerizations, the necessity for using a plurality of reactors to achieve the desired result increases energy consumption, raises production costs, and requires greater investment in equipment and facilities. A single reactor process for the polymerization of olefins to provide polyolefins, including propylene homopolymers and propylene copolymers having improved melt flow rates and molecular weight distributions together with high tacticity, would thus provide significant advantages and be a substantial improvement in the art.